The present invention relates to an apparatus for welding, and in particular to a welder adapted to weld components of a material having a relatively sharp melting point, such as cathodic and anodic lead alloy plates in some types of batteries.
Robotic welders are used in a wide variety of applications to facilitate automated welding in assembly lines. These robotic welders typically include a robot, a welder mounted thereon, and a controller in communication with the robot and controlling the location and movements thereof. However, robotic welders are often not able to position welds as accurate as may be desired due to product variations, locational variations due to inconsistent fixturing, and variations caused by conditions inherent in the robot, such as wear and play in joints and members of the robot's moveable arm. In batteries, this can be problematic since anodic plates (or cathodic plates) in adjacent cells of a battery must be welded together through a hole in a cell-separating wall in a battery casing. If the weld tips are not very accurately located at a center of the hole during welding, the welded material to be welded heats non-uniformly. Since the plates are made of a lead or lead alloy having a relatively sharp melting point, uneven heating of a weld causes "hot" spots to occur that spit and splatter liquid material, while "cool" spots do not weld properly for good electrical contact and current flow.
AC welders have been employed to create welds on the anode or cathode components of a battery. However, there are several problems with AC welders, particularly when used on materials having a sharp melting point where close and accurate control of the welding process is required. For example, AC welders typically use a single phase power that can result in an unbalanced line power. AC welders require a large amperage disconnect switch separating them from the main power supply. AC welders operate on a relatively low power factor, thereby increasing the energy costs associated with running these welders. Also, AC welders typically operate at about 60 Hz making the energy input into the associated weld relatively imprecise, which can prove problematic in precision welding applications. AC welders create a high impedance because of the reversing magnetic fields building and collapsing when operating within the 60 Hz range. Exemplary of this is a typical situation in which a source voltage to the AC welder is about 10 VAC (voltage alternating current), it is considered sufficient when only one VAC is actually supplied to the weld point. This example illustrates the significant voltage loss. Another problem associated with the use of an AC welder is that the reversing magnetic fields as produced in an AC weld secondary loop make the loop sensitive to magnetic materials. When an excessive amount of magnetic material accumulates on the associated clamps and tilling within this loop, the loop will become saturated, resulting in failure to obtain the desired weld current, thereby resulting in an imprecise or uneven weld. Yet another drawback associated with AC weld systems is the size and relative weight of the AC weld system transformer. As an example, an AC weld system transformer capable of delivering 22 kA (kilo amps) weighs approximately 200-300 pounds, which is large, bulky, and difficult to deal with around a robot since the robot requires significant open space to operate. Another problem associated with AC welders is the fact that most metals become magnetic when molten. As a result, the oscillating magnetic field of the AC welder tend to create a condition where the molten metal is expelled from the weld joint, thereby resulting in an inadequate weld joint or contamination of components surrounding the weld joint. Finally, the AC wave form of an AC welder cannot be easily controlled, and therefore it does not allow the fine tuning wanted and necessary in precision welding applications.
Specific to welding processes as associated with the manufacture of batteries, it is often difficult to provide for an exact alignment between the precise location of the battery casing and anodic (or cathodic) components at which point the weld is to be located. One reason is because the rigid construction of the weld heads results in a large weld-tip-holding mechanism and frame that is heavy, massive, and difficult to manipulate with accuracy. More specifically, the rigid construction of the weld heads result in spatial and physical limitations that prevent the weld heads from reacting to correct misalignment conditions between the welding tips of the weld head and the battery components being welded. Further, many welding machines are difficult to access during routine maintenance. Along the same lines, precise fixturing of the battery to be welded is highly important due to the precise nature of the welds in the exact locations thereof.
Accordingly, a welding apparatus is desired solving the aforementioned problems and having the aforementioned advantages, especially for the purpose of robot welding materials having relatively sharp melting points such as lead alloy components used in batteries.